The origin FeatureTable on which you are performing related query operations should be in LOADED state. You can check it using `featureTable.getLoadStatus() `

If you have set the map from MobileMapPackage to MapView, that should have LOADED all the layers and tables in the map. If you are working without a MapView then you may have to explicitly load the origin table on which you want to perform related query operations

You can also view the metadata of RelationshipInfo's by calling `ArcGISFeatureTable.getLayerInfo.getRelationshipInfos()` just to make sure the relationships do exist in the MMPK

If it is still returning empty related tables, then we may have to check the data and the relationship details in ArcGISDesktop where the mmpk is generated.